1. Field of the Invention
This invention relates to a correction processing device for matching the luminescent color of a cathode-ray tube (CRT) to an external color standard, such as the print color of a printer, and particularly to a CRT calibration device for transmitting a signal representing the individual color reproduction characteristics of different CRTs to a printer.
2. Description of Related Art
There has been a remarkable difference in visual appearance between the luminescent color of a CRT and the print color of a color printer. This arises because of the difference in coloring principle and the difference in primary colors of these devices. Therefore, it is necessary to match the color of the CRT and the color of the printer with each other. However, the color reproduction characteristic differs greatly between different CRTs. Thus, a user must recognize the color reproduction characteristic of the CRT, when a color displayed on the CRT is to be accurately printed on a printer.
Red (R), green (G) and blue (B) are the three primary colors of a CRT. A gradation reproduction characteristic .gamma. representing the relationship between the gradation signal value of each of the RGB color points is one factor for determining the color reproduction characteristic of the CRT. The gradation reproduction characteristic of the CRT can be approximated by Eq. 1: EQU V=(N/255).sup..gamma. ( 1)
where V is the relative luminance value with 0.ltoreq.V.ltoreq.1 when the maximum luminance value is set to 1, N is the gradation signal value, where 0.ltoreq.N.ltoreq.255, and .gamma. is the gradation reproduction characteristic.
The .gamma. value is representative of the gradation reproduction characteristic. The .gamma. value can be calculated based on a gradation signal value B, where 0.ltoreq.B.ltoreq.255, and a relative luminance value A, where 0.ltoreq.A.ltoreq.1, by Eq. 2: EQU .gamma.=log(A)/log(B/255) (2)
By supplying this .gamma. value to the printer, the color output by the printer can be adjusted based on the gradation reproduction characteristic of the CRT. Therefore, a device for checking the .gamma. value of each CRT is incorporated to the apparatus, and this device is called a CRT calibration device.
FIG. 9 shows one type of CRT calibration mechanism. A color patch A whose relative luminance value is known and a color patch B whose gradation signal value is adjustable by an user are displayed aligned with each other on a CRT to be calibrated. Thereafter, the gradation signal value of the color patch B is adjusted by the user until the color patch A and the color patch B appear to be the same color. That is, the color patches A and B become visually coincident with each other. The gradation signal value when these patches A and B look the same is indicated. Representing the relative luminance value of the color patch A and the gradation signal value of the color patch B when they appear visually coincident by VA and NB, respectively, the .gamma. value is calculated according to Eq. 3: EQU .gamma.=log(VA)/log(NB/255) (3)
The calculation of the relative luminance value of the color patch A is based on the following technical idea for area gradation. A black dot for which the gradation signal is set to "0" and a white dot for the gradation signal value is set to "255" are mixed with each other to calculate the relative luminance value. Now, representing the area (occupation) rate of the gradation signal value 255 by m and representing the relative luminance value of the gradation signal value 255 by P, Eq. 4 is satisfied: EQU VA=m.times.P (4)
The gradation signal value 255 represents the maximum luminance value. Thus: EQU P=1 (5)
Therefore, the relative luminance value of the color patch A is equal to m. Thus, it is a known value irrespective of the characteristic of each individual CRT. Therefore, the .gamma. value can be obtained from Eq. 3.
When the color points of the gradation signals values of 0 and 255 are arranged at a high spatial frequency in a scanning direction on a CRT, the ability of the CRT to respond is lowered, and Eq. 5 (P=1) is not satisfied, as shown in FIG. 10. Therefore, the relative luminance value VA of Eq. 3 cannot be calculated. Thus, the .gamma. value cannot be calculated.
On the other hand, a lateral-striped pattern as shown in FIG. 11 is considered as a method of lowering the spatial frequency of the black and the white in the scanning direction to solve this problem. However, this pattern is still recognized only as a lateral-striped pattern, so that it is difficult for an user to recognize this lateral-striped pattern as a uniform gray color. Therefore, it is difficult to match the colors of the color patches A and B with each other, and thus it is impossible to perform the color matching.